The quantity of the second messenger inositol trisphosphate required to initiate activation of sea urchin eggs by direct injection is known to be about 4 x 10(-18) moles. We have previously found that, at the time of fertilization, a single sea urchin spermatozoon contains approximately that amount of inositol trisphosphate. This suggests that a single spermatozoon may insert enough inositol trisphosphate into the egg to initiate activation. Thus, inositol trisphosphate may act as a primary messenger in the fertilization process. This possibility would seem even more likely if the process of insertion by the spermatozoon were more efficient than insertion by injection. We have now calculated the relative efficiencies of these two modes of insertion and found that insertion by the spermatozoon is about three times more efficient than insertion by injection.